In this day and age, every website should be optimized for mobile viewing. However, with so many devices available on the market, things can be a little tricky — unless you use a tool like Mobile Emulator.
Visit http://www.page2images.com/mobile_phone_emulator, paste the link to your website in the URL box, choose the device, screen size / orientation, and click the Submit button. The resulting screenshot will appear below. You can then download it.
Note that each screenshot also has a dedicated link for easy sharing.